Mobile CS (Circuit Switched) services based on GSM (2G) and WCDMA (3G) radio access are a world-wide success story and allow obtaining telecommunication services with a single subscription in almost all countries of the world. Also today, the number of CS subscribers is still growing rapidly, boosted by the rollout of mobile CS services in dense population countries such as India and China. This success story is furthermore extended by the evolution of the classical MSC architecture into a softswitch solution which allows using packet transport infrastructure for mobile CS services.
In 2006 the 3GPP group started with a work item called “Evolved UTRA and UTRAN” which became commonly known under the acronym E-UTRAN, Evolved Universal Terrestrial Radio Access Network. The purpose of the work item was to define a Long-Term Evolution (LTE) concept that assures competitiveness of 3GPP-based access technology.
LTE/E-UTRAN will use OFDM radio technology in the downlink and SC-FDMA for the uplink, allowing at least 100 Mbps peak data rate for downlink data rate and 50 Mbps for uplink data rate. LTE radio can operate in different frequency bands and is therefore very flexible for deployment in different regions of the world.
In parallel to Radio Access Network (RAN) standardization for the LTE, 3GPP also drives a System Architecture Evolution (SAE) work item to develop an evolved packet core network. The SAE core network is made up of core nodes such as Control Plane nodes as the MME, Mobility Management Entity and User Plane nodes as the Serving Gateway (S-GW) and the Packet Data Network Gateway (PDN GW or P-GW). A co-location of the S-GW and the P-GW is also denoted Access Gateway (AGW).
Common to LTE/SAE is that only a Packet Switched (PS) domain will be specified, i.e. all services are to be supported via this domain. GSM and WCDMA however provide both PS and CS access simultaneously. So if telephony services shall be deployed over LTE radio access, an IMS based service engine is mandatory.
It has been investigated how to use LTE/SAE as access technology to the existing CS core domain infrastructure. The investigated solutions are called “CS over LTE” solutions. Three different solutions have been identified so far.
The first solution is called “CS Fallback” and means that a mobile terminal is performing SAE MM (mobility Management) procedures towards the MME while camping on LTE access. The MME registers the terminal in a MSC-S for CS based services. When a page for CS services is received in the MSC-S it is forwarded to the terminal via the MME and then the terminal performs fallback to the 2G or 3G RANs. Similar behavior applies for Mobile originated CS services and when these are triggered and the terminal is camping on LTE access, it will fallback to 2G or 3G RANs and trigger the initiation of the CS service there. This solution has been specified in the technical standard 3GPP TS 23.272.
The second solution is called CS over LTE Integrated (CSoLTE-I). In this solution the same SAE MM procedures as for “CS Fallback” are used, but instead of performing fallback to the 2G or 3G RANs, the terminal will perform all the CS services over the LTE access. This means that the CS services (also called Connection Management, CM procedures) are transported over IP-based protocols between a Packet MSC PMSC and the terminal using the LTE access and the SAE nodes like the AGW.
The third solution is called CS over LTE Decoupled (CSoLTE-D). In this case both MM and CM procedures are transported over IP-based protocols directly between the PMSC and the terminal using the LTE access and the SAE user plane nodes like the AGW.
3GPP has also standardized the Generic Access Network (GAN) concept starting from 3GPP Release-6. The more correct name is “Generic Access to A/Gb Interfaces” and this standardization was based on the Unlicensed Mobile Access (UMA) de-facto specifications.
GAN provides a new radio access network and the node corresponding to the GERAN (GSM EDGE Radio Access Network) the Base Station Controller, BSC is called Generic Access Network Controller (GANC). GAN is specified in the 3GPP TS 43.318 and TS 44.318. The basic principle is that the mobile terminal (in the specifications called MS, Mobile Station) is a dual-mode radio handset including for example both WiFi and 3GPP-macro radio support (GSM, WCDMA or both). The mobile terminal connects to a WiFi Access point (AP) using the WiFi Radio. The GAN standard defines for example how the mobile terminal can function in GAN mode and access the services provided by the GSM CN (Core Network) using the Up-interface between the mobile terminal and the GANC.
The main principle in GAN is that the mobile terminal is configured with Provisioning GANC address information and this is the initial point of contact in the network and the mobile terminal triggers the GAN Discovery procedure towards the Provisioning GANC that is placed in the Home Public Land Mobile Network HPLMN. The only purpose of the GAN Discovery procedure is to provide the mobile terminal with information about a Default GANC that also resides in the HPLMN. The Default GANC is the node where the mobile terminal always connects to initially when it attempts to use GAN in a new location. The Default GANC may redirect the mobile terminal to a Serving GANC that may be placed either in the HPLMN or in a VPLMN (Visited PLMN). The mobile terminal may also store information about Serving GANCs in a Serving GANC table.
The main principle in the CS Domain Control Plane Architecture related to GAN and the Up-interface is that the GANC uses the normal A-interface signaling towards the MSC. The GANC interworks the related protocol, like BSSAP, towards the relevant GAN-protocols, like GA-CSR (Generic Access, Circuit Switched Resources), in both directions.
The solution of using the GAN concept for CS over LTE (CSoLTEvGAN) is disclosed in the 3GPP technical report TR 23.879 as one of the alternatives for CS service support over LTE. The technical report covers a number of different alternatives. The basic idea for the CSoLTEvGAN alternative is to see LTE as a Generic Access Network and to use the GAN protocols for control and user plane.
An important difference in the CSoLTEvGAN solution compared to the GAN solution is the registration procedure. It has been proposed that the mobile terminal triggers the GAN registration procedure when the mobile terminal enters LTE coverage and triggers the GAN registration update procedure each time the mobile terminal changes tracking area (TA) in LTE/E-UTRAN. At each GAN registration and registration update procedure the mobile terminal will include cell identification information about the LTE/E-UTRAN cell the mobile terminal is connected to. The GANC will, based on the E-UTRAN cell identification information, select a GAN cell having its own cell identification information. This GAN cell identification information is sent to an MSC in the core network in order to indicate the current location of the mobile terminal. The relations between the E-UTRAN cell identification information and the cell identification information for the GAN cell is configured in the GANC. The solutions proposed so far rely on the GANC being manually configured.
A disadvantage is that a manual configuration of the needed relations may become a tremendous burden for the operator. Initially, there needs to be one GAN cell defined for each E-UTRAN cell and for each MSC where the mobile terminal may connect to via a GANC. After this, whenever there are changes to the cell structure in the E-UTRAN, the effect of the related changes needs to be verified also in the GAN to see whether any changes are needed to the defined relations between E-UTRAN and GAN cells.